1. Field of the Invention
The present invention relates to a zoom lens for use in a TV camera, a video camera, a digital camera, and a monitoring camera, for example, and an imaging device using the zoom lens.
2. Description of the Related Art
A zoom lens including, in order from an object side to an image surface side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power is known (refer to Patent Document 1: JP H10-054937A). In such a zoom lens, the second and third lens groups are moved such that the second lens group is positioned on the most object side at the wide-angle end and the third lens group is positioned on the most image surface side at the telephoto end during zooming.
As such a zoom lens, a zoom lens including a diffraction optical element in the first lens group is known (refer to Patent Document 2: JP 2008-197534A).
As such a zoom lens, a zoom lens, which compensates a chromatic aberration to a near-infrared region, without including a diffraction optical element is also known (refer to Patent Document 3: JP 2008-241884A).
Various types of zoom lenses have been developed for TV cameras. For example, as a zoom lens suitable for a high zoom ratio, a zoom lens is known including, in order from an object side to an image surface side, a first lens group having a positive focal distance, a second lens group having a negative focal distance, a third lens group having a negative focal distance, and a fourth lens group having a positive focal distance. In such a zoom lens, the second and third lens groups are moved during zooming.
In such a zoom lens, the second lens group operates as a variator for zooming, and the third lens group operates as a compensator for compensating fluctuation in image surface position associated with zooming. However, in order to downsize a zoom lens and obtain a high zoom ratio, the third lens group sometimes performs a part of a zooming operation.
In Patent Document 1, the third lens group performs a part of the zooming operation, but the zoom ratio is less than 20:1.
A monitoring TV camera may perform imaging having sensitivity to a near-infrared region of a wavelength of about 900 nm or below. Such a monitoring camera obtains an accurate color image only with visible light by cutting near-infrared light during daytime with sufficient light volume. On the other hand, such a monitoring camera transmits all of light from a visible region to a near-infrared region to obtain a light volume in a poor weather, twilight, or dawn, and projects and illuminates infrared light of a wavelength of about 850 nm by cutting visible light during night time.
A zoom lens for use as a photographing lens is therefore required so that a chromatic aberration is compensated not only to a visible region but also to a near-infrared region. When the chromatic aberration is not compensated to the near-infrared region, it is necessary to refocus upon switching between visible light and near-infrared light, or a sufficient resolution cannot be obtained in the case of transmitting all of light from the visible region to the near-infrared region.
In this type of zoom lens, in order to preferably compensate a chromatic aberration, extra-low dispersion glass as represented by FPL 51 (manufactured by OHARA CO., LTD.) or FPL 53 (manufactured by OHARA CO., LTD.) is used for the first lens group in which an axial marginal light beam height is increased on a telephoto side and the fourth lens group in which an axial marginal light beam height is increased on a wide-angle side. A diffraction optical element in the first lens group so as to compensate the chromatic aberration by using negative dispersion of the diffraction optical element has also been considered.
In Patent Document 2, a chromatic aberration is compensated by using the diffraction optical element in the first lens group. However, a zoom lens having a zoom ratio of more than 25:1 has a large telephoto ratio (ratio of total length of lens relative to focal distance) of about 1.0 at the telephoto end, so that the zoom lens is not significantly downsized.
In Patent Document 3, an extra-low dispersion glass is used for the first lens group and the fourth lens group, and aberrations are compensated to the near-infrared region. However, the zoom ratio is about 22:1, which is smaller than 25:1.
Users have many demands for TV cameras or video cameras. Users always request high quality images and downsized cameras. For this reason, both of a high performance and downsizing are desired for the zoom lenses. A relatively high zoom ratio is also desired.